The coating of metallic materials, such as copper, steel or aluminum, with metallic tin or tin alloys, especially tin-lead alloys, is widespread in numerous applications in electroplating. The objective of the coating may be, for example, to afford protection to the substrate against corrosion and tarnishing, to enhance soldering properties, or to improve the slip properties of the metal surface. In order to be able to apply such an alloy coating, either with or without electric current, it is necessary to have stable solutions of the requisite metal salts in a relatively wide concentration range for the formulation of the coating bath. And, indeed, this is the case for tin and lead and other common alloy components.
Against the background of the increasing recovery of the metallic constituents from electrical or electronic components, the importance is gaining of those electrocoating processes which can be used to produce toxicologically unobjectionable alloys. The replacement of lead by another metal in tin alloys is the subject of diverse efforts. Against this background, the metal bismuth is attracting particular interest as a partner to tin in alloys. As a result, tin-bismuth alloys prepared by metallurgical means are now already being used to an increasing extent as a replacement for toxicologically objectionable tin-lead alloys.
Suitability for use as an electrocoating for material is possessed above all by those metal salts which can be dissolved in water easily and in almost infinitely adjustable concentrations. Particular preference is given to aqueous solutions, since electrocoating baths are, more and more, being regulated by means of automatic metering pumps.
In terms of the practical use of bismuth salts as a constituent of alloying baths, these properties lead to considerable problems in that they considerably restrict the use concentrations and the scope for formulation. In electrocoating baths, the Bi contents are well below 100 g/l along with very high acid contents at the same time. In an electrocoating bath which is used for producing an alloy coat there is no need for a high metal concentration. For products intended for use as raw material for electrocoating baths, however, it is indeed an important characteristic. High metal salt concentrations are important so that not too much water is introduced into the electrocoating bath when the metal content is refreshed.
Nevertheless, a number of bath formulations are known. For the electrodeposition of bismuth on aluminum and steel, a bath is described that is based on bismuth perchlorate or bismuth chloride with the sodium salt of ethylenediaminetetraacetic acid (Na.sub.2 EDTA) (Eidenschink, Dommain, Metalloberflache [Metal surface], Volume 20, 1966, pp.173/174). The bismuth perchlorate is obtained by dissolving basic bismuth carbonate in excess perchloric acid or by anodic dissolution of metallic bismuth.
The electrodeposition of tin-bismuth alloys on the basis of tetrafluoroboric acid electrolytes is described by Drescher et al. (Galvanotechnik, 85 (1994), 4187-4191). The alloying electrolyte is obtained by dissolving bismuth oxide or bismuth carbonate. Enrichment by anodic dissolution of bismuth at high current densities is a further possibility.
JP 08246200 A teaches the preparation of tin(II) sulfate and bismuth sulfate by the electrolysis of solutions in sulfuric acid, with tin or bismuth anodes, using an anion exchange membrane.
In Patent Abstracts of Japan, C-730, 1990, Vol. 14, No. 282.JP-2-88790A describes the preparation of aqueous electroplating solutions based on bismuth/tin compounds. This entails subjecting acids such as sulfuric acid or organic sulfonic acids, an example being an alkanesulfonic acid, to electrolytic dissolution of the anode using metallic bismuth as anode. In this case, the bismuth ions removed from the solution by the electrocoating process are replaced continuously by the bismuth ions forming at the anode, do that a virtually constant concentration of bismuth ions is established which allows a stable electrocoating process.
EP 0 715 003 A teaches that stable baths for depositing a tin-bismuth alloy based on solutions in methanesulfonic acid can be prepared by adding various complexing agents, such as gluconic acid, citric acid and others, although the origin of the Bi content is not specified.
WO95/25008 describes the use of electrolytes containing bismuth chloride for the coating of copper, where the bismuth is introduced in the form of the oxide and it is necessary to ensure a large excess of acid.
Preparation of salts takes place in accordance with the prior art by dissolving bismuth oxide or basic bismuth carbonate in the concentrated acid. When carrying out this process it is necessary to ensure that there is always a sufficiently large excess of acid present, since, when dissolved in water, bismuth salts of strong inorganic acids undergo immediate hydrolysis in accordance with the following equation: EQU BiCl.sub.3 +H.sub.2 O.fwdarw.BiOCl+2HCl
A similar reactivity has been described for the sulfates and nitrates of bismuth as well (see e.g. F. A. Cotton, G. Wilkinson, Anorganische Chemie [Inorganic Chemistry], 4th edition, Verlag Chemie, pages 449-494). If a bismuth salt solution prepared in concentrated acid is diluted with water or another electrolyte, hydrolysis again takes place.
For example, a stable solution of bismuth sulfate can be prepared only by dissolving Bi.sub.2 O.sub.3 in 50% strength sulfuric acid. An attempt at dissolving it in dilute sulfuric acid, such as 10% strength acid, leads directly to bismuthyl sulfate, which is obtained as an insoluble precipitate.
Electrocoating processes based on the use of highly concentrated bismuth salt solutions are unknown. As demonstrated by the examples, the starting point is either solid oxides or carbonates, which leads to increased manual effort, or the desired bismuth concentration is generated in situ. Generating the bismuth content in situ also has disadvantages. Firstly, soluble anodes as a source of Bi are vital, which on the one hand necessitates their production and recycling and on the other hand leads to the formation in the electroplating bath of insoluble fractions, which may be highly disadvantageous for the quality of coating. Furthermore, the soluble anodes may not be dimensionally stable, which leads to a change in the current density distribution in the electroplating bath and so likewise to undesirable effects on the thickness distribution of the electrodeposited coat.
Processes for preparing highly concentrated bismuth salt solutions, especially those based on methanesulfonic acid, which can then be diluted in use to the desired concentration, on the other hand, are not known.
WO 93/07309 teaches that using bismuth anodes and tin-containing electrolytes based on methanesulfonic acid it is possible by anodic oxidation to introduce bismuth ions into the electrolytes, the Bi content remaining in the order of 10 g/l.